Fiberoptic probes are designed and utilized with light-emitting diode oximeter instruments for use in neonatal intensive care unit monitoring. A small-diameter fiberoptic probe is passed down the lumen of an umbilical artery catheter. Arterial oxygen saturation is sensed continuously and dye curves sensed on demand. The continuous oxygen saturation monitoring and repeated dye curves will help identify the physiological cause of hypoxia in newborns and to evaluate and quantify the effectiveness of therapeutic interventions. The dye curves will be used to distinguish between intrapulmonary and extrapulmonary shunts and therefore guide the clinician in theoretical modalities. Preliminary studies will be performed in neonatal lambs. Computer modeling algorithms will be applied to the dye curves to calculate shunt flow and cardiac output.